The invention relates to diffractive elements, and relates in particular to volume holograms. Volume holograms may be used for a variety of uses, including data storage, imaging, inter-connections for optical circuits, neural networks and communication systems etc. A variety of materials have been used for forming conventional volume holograms, including photorefractives and photopolymers. For example, U.S. Pat. No. 5,858,614 discloses the use of a polymerizable compound that is liquid at ambient temperature. Conventional recording materials however, may not be readily changed once a diffractive pattern or hologram has been recorded on them. In some cases, the original recording is permanent, while in other cases the diffractive pattern may be changed by erasing the old pattern and recording a new one. This is a time-consuming process and difficult to update the information content stored in the hologram.
Tunable diffraction gratings may be potential candidates for video-rate applications. The grating light valve (GLV by Silicon Light Machines of Sunnyvale, Calif.) is an example of a tunable grating. Such a device typically includes adjacent ribbons that may be actuated between two positions to provide either a reflective surface (in which the ribbons are aligned with one another) or a diffractive surface (in which alternate ribbons are displaced by ¼λ). While such grating light valves provide directional (e.g., zero and first order etc.) switching, they are not suitable for use in volume holograms because of resolution limit.
There is a need therefore, for a volume diffractive material that may be changed from a first volume diffractive pattern to a second volume diffractive pattern without requiring the erasure and the re-recording of a new diffractive pattern.